1. Field of the Invention
The present invention relates generally to a cooled mirror structure, and more particularly pertains to a cooled mirror structure useful primarily in optical systems having lasers which generate extremely high energy flux levels. In these optical systems a mirror or optical reflector is commonly utilized to reflect an incident laser beam in a desired direction, and despite attempts to construct such mirrors to reflect almost all of the incident laser beam, a small portion thereof is absorbed by the mirror, which can be a substantial total quantity of energy because of the extremely powerful laser beams involved in these systems. The absorbed optical energy is converted to thermal energy which is transferred through the reflector by conduction. The thermally induced temperature increase in the reflector establishes thermal compressive stresses therein which distort the shape or linearity of the reflector front surface, causing errors in the direction of the reflected optical wavefront. Accordingly, it is often necessary to cool the mirror structure to minimize distortions in the reflective surface while accommodating extremely powerful and intense laser beams.
2. Discussion of the Prior Art
The prior art has evolved some highly specialized structures to combine efficient cooling with stable support for the reflecting surface. One approach taken by the prior art to dissipating the thermal energy absorbed by the reflector front surface is to incorporate fins or radiators on the back surface thereof. The fins or radiators serve as heat sinks which dissipate the absorbed thermal energy but do not effectively prevent reflector front surface distortions.
Other prior art cooling arrangements force a fluid or coolant to flow under relatively high pressure against the back surface of the reflector. Examples of some of these prior art cooling arrangements are disclosed in Griest U.S. Pat. No. 3,781,094, Simmons et al. U.S. Pat. No. 4,053,241 and Weiss U.S. Pat. No. 3,932,029.
Rambauske et al. U.S. Pat. No. 3,841,737 is of interest as this patent discloses a mirror structure wherein a metallic base has a relatively low coefficient of thermal expansion, and the base includes cooling passageways through which a liquid refrigerant is passed and evaporated therein in a traditional evaporative cooling process.
Glickler et al. U.S. Pat. No. 4,099,853 is also of interest by disclosing a reflector for a high power laser beam in which a reflector element having a low absorption, transmitting substrate is provided with a rear reflective coating. The reflector element is mounted within a housing using low stress supports to prevent deformation of the reflective element and a resulting loss in optical beam quality in the reflected laser beam. Coolant is sprayed onto the rear surface of the reflector element, preferably in a pattern which modulates the coolant flow volume in accordance with the intensity distribution across the incident laser beam to maintain uniform heating in the reflective element and prevent distortions in the reflected beam.
However, even the aforementioned highly specialized cooled mirror structures are not capable of handling presently available laser energy flux levels, particularly continuous wave operations for one second and greater time durations, while maintaining surface distortions within acceptably low limits.
In general, the aforementioned prior art approaches have concentrated on cooled mirrors having metal substrates which are designed to operate at or near room ambient temperature. The thermal distortions induced in the mirrors are determined by the characteristics of the substrate material at the ambient temperature, namely the ratio of its coefficient of thermal expansion .alpha. to its thermal conductivity k. For some metals and ultra-low expansion materials such as ULE.RTM. (ULE is a trademark of Corning Glass Works for their code 7971 titanium silicate glass) and CER-VIT, the .alpha./k values are of the order of 10.sup.-8 meters per watt. However, these materials still yield intolerably high thermal distortions in laser systems having actively cooled mirror elements.